A large number of products using variable wavelength lasers have recently been developed. Lasers have been developed with the object of applications to biochemical measurements and semiconductor measurements in a range from ultraviolet radiation to the visible light and medical measurements in a range from the visible light to near-infrared radiation. Products using specific features of lasers, such as a coherence characteristic and a high-output characteristic, have been developed, but stabilization of laser oscillation at a high output remains to be a basic requirement.
A warm-up process of inducing laser oscillation for several tens of minutes is typically performed to stabilize the laser output. As a result of the warm-up, the temperature of optical members constituting the resonator, a laser medium, an excitation medium, and cooling water is stabilized in a thermal equilibrium state. Patent Literature 1 (PTL 1) discloses a method for determining the laser preparation completion state in which the warm-up process of causing laser oscillation is performed in a stepwise manner and the laser output is stabilized.
In a Nd:YAG laser with flash lamp excitation, a comparatively stable laser output can be obtained by using only light emission from the flash lamp, that is, without causing laser oscillation. The temperature of the Nd:YAG rod disposed adjacently to the lamp rises due to the absorption of energy generated by light emission from the flash lamp, a thermal equilibrium state is attained, and the laser output is stabilized.
Patent Literature 2 (PTL 2) discloses a method for stabilizing laser output by performing temperature control of a KTP crystal, which is a nonlinear optical crystal, by using a Peltier element in a laser generator using the KTP crystal.
A variable wavelength laser with titanium sapphire (Ti:sa) or a dye as a laser medium uses a basic wave of Nd:YAG or the like or a harmonic thereof as an excitation light source and generates laser radiation by causing selective resonance of the wavelength at which the generation is possible. In order to stabilize the generated laser output, it is important to stabilize the laser output of the excitation source and also adjust the temperature of the laser medium (Ti:sa or a dye) to a thermal equilibrium state same as that during laser oscillation.
A Non-Patent Literature 1 (NPL 1) discloses a photoacoustic measurement apparatus as a medical diagnostic apparatus incorporating a variable wavelength laser in which the presence of a tumor inside a breast mass is diagnosed by using a photoacoustic effect. In the photoacoustic measurement apparatus, a measurement site is irradiated with nanosecond pulse laser radiation, ultrasound waves generated in the measurement site are received, and the received signal is analyzed, thereby obtaining an image. By using a wavelength-variable laser such that uses Ti:sa, it is possible to obtain tissue information that is based on the difference in absorption coefficient between the body tissues. Since photoacoustic measurements have a low degree of invasiveness, the measurements can be repeatedly performed, while changing the measurement site. Further, the measurement interval of one cycle is short and the laser generation time is comparatively short. Therefore, laser oscillation is most often induced intermittently and it is important to stabilize the laser output.